The invention refers to the combination of a circulation pump and a resistance heater to heat up and/or to maintain the temperature of a body of fluid. While the heat input into a water circuit does not create any problems because the high heat capacity of the water allows heating elements with very high performance per unit of surface, heating devices for heating up and subsequently maintaining the temperature in photochemical baths create major problems, because too high wall temperatures of the heating elements may cause decomposition of the sensitive chemicals. Some developing solutions can only stand excess temperatures of 6 K., which is only achievable with a very low power density. Such low wall temperatures require large heating coils. The casing of these heating coils leads to large fluid cross sections which make it impossible to avoid dead zones, zones in which the relative velocity of the fluid is zero. Since the supply of energy is distributed equally over the length of the heating element, the unwanted overheating will appear in such dead zones. With the increase in length the heat capacity of the heating coil increases which complicates the temperature control.
There will always be a temperature distribution at the exit area which reflects the temperature difference between the current paths which flow laminar due to the low velocity. This means that the temperature sensor which is installed to keep the temperature constant may be exposed to a current path which is either too hot or too cold. In developing baths which allow only a temperature difference of 0.1 K. this can be extremely harmful.
One finds analogous conditions in holding the temperature of baths constant, especially in medical bathroom installations even if the requirement for a constant temperature here allows a range of variation of 1 K. To a certain extent also in this application the overheating is harmful because calcareous deposits are formed in the hottest areas whereby the heating coil in the course of time will be covered with an insulating layer of scale which can lead to destruction of the heating coil by local overheating and thus to a connection between live parts and the bath water.
In the circulation of coolants it is also important to have only a slight excess temperature because the evaporation enthalpy of coolants lies by tenth power under the one of water. In overheated areas therefore vapor develops which creates loud noises in the system and which can lead to further local overheating of the heating elements and thus, at higher overheating temperatures, may cause decomposition of the coolant.
The invention avoids these drawbacks. According to the invention the circulation flow to be heated will be conveyed through a tube which contains a tubular resistance heating element which is closed on one side. The surface of the tubular heating element and the inner wall of the surrounding tube form an annular clearance of small radial extension. In this annular clearance all current paths experience a uniform and equally high temperature increase. Thereby a local overheating is absolutely eliminated. Due to this uniform heating the temperature sensor can be attached to the tubular heating element whereby a good heat conductivity of the connection is important. At the outlet of the tube a circulation pump is arranged. This makes it possible to mount the temperature sensor and the necessary safety thermostat close to the electrically driven pump whereby cables along the tube can be avoided. It has been found that the mounting of the pump on the suction side leads to an extremely uniform velocity of the current paths which run along the tubular resistance heating element.
The invention will be described with the help of figures.